1. Technical Field
The present invention relates to a welding tip for arc welding that includes a contact having an insulating guide which serves to increase a wire extension length measured from a contact point, and a metal guide which accommodates therein the insulating guide. More particularly, the present invention relates to a welding tip capable of maintaining a stable electric contact between a contact and a welding wire to stably generate arc between the welding wire and a base material, and relates to an arc welding apparatus provided with a welding tip of this kind.
2. Related Art
An arc welding apparatus is configured to feed a welding wire from a wire supplying apparatus toward a contact of a welding tip mounted to a welding torch and to supply welding current to the welding wire through the contact, whereby welding is performed by utilizing arc heat produced between a base material and the welding wire.
A consumable electrode type gas shielded arc welding, which belongs to arc welding, is performed with use of energy produced by arc that is generated between a base material and a continuously supplied welding wire, while enveloping the arc by a shield gas which is carbon dioxide gas, argon gas, or a mixture thereof. Arc welding of this kind is widely employed for the automatic or semi-automatic welding of thin plates, such as suspension component welding performed by welding robots in automotive industry.
In consumable electrode type gas shielded arc welding, an amount of fused welding wire increases with the increase in welding current, and therefore, the welding current is increased in order to improve the fusing speed of the welding wire or the welding speed. On the other hand, the increased welding current can increase a weld penetration depth in a base metal, resulting in burn-through in the thin plate welding. To obviate this, it is proposed to mount an insulating guide of a heat-resistant insulating material such as ceramic to a distal end of a contact through which the welding current is supplied to a welding wire, in order to increase the wire extension length, i.e., the distance between the welding tip and the base metal, thereby increasing Joule heating in a wire portion corresponding to the wire extension length. By doing this, an amount of fused wire increases, so that the welding speed may be improved while preventing the burn-through of the base material.
However, a welding torch having an insulating guide at a distal end of a contact cannot maintain a constant contact point, i.e., a constant feeding point between the welding wire and the contact, causing unstable power supply which results in an unstable arc.
As for a welding torch having no insulating guide, a welding wire having a habit of being curled can freely be bent after passing a wire insertion hole formed in a contact, and is thus positively in contact with the contact at an extreme end of the wire insertion hole, whereby stable power supply can be achieved. Contrary to this, in a welding tip having an insulating guide provided at a distal end of a contact, a welding wire is prevented from being freely bent at the extreme end of the contact and is prohibited to be freely bent until it passes through the insulating guide. Thus, the welding wire is prevented from contacting with the extreme end of the contact. This makes the contact point unstable and hence the power supply tends to be unstable. Further, such a variation in contact point causes local fusion of the feeding tip and the formation of an oxide layer therein, so that the contact is liable to be consumed. Though a relatively stable arc is obtainable at an early stage of usage of the contact, the arc becomes unstable with the advance of the contact being consumed.
FIG. 1 shows an example of a welding tip for arc welding that includes an insulating guide, having a central portion thereof formed with a guide hole for guiding a welding wire, and a metal guide for holding therein the insulating guide.
The welding tip shown in FIG. 1 includes a contact 200 having a male thread portion 202 thereof threadedly engaged with a female thread portion 206 of a welding torch body 204. The contact 200 is formed with a feeder hole 216 at which the contact is in contacts with a welding wire W and electric power is supplied to the wire. The welding tip further includes a cylindrical metal guide 210 having a female thread portion 212 thereof threadedly engaged with a male thread portion 214 of the contact 200. Further, a cylindrical ceramic insulating guide 218 is retained inside the metal guide 210 and formed with a guide hole 220 for guiding the welding wire W. In FIG. 1, reference numeral 208 denotes a conduit tube inserted into the welding torch body 204, and symbol A denotes an arc generated between the welding wire W and a base material.
The welding tip shown in FIG. 1 can increase the extension length of the welding wire W since the contact 200 is provided at its distal end with the metal guide 210 and the insulating guide 219. The increased wire extension length can increase an amount of heat (I2R) generated by electric current flowing through the tip end portion of the welding wire A, which portion extends from the contact point (feeding point) between the welding wire and the contact 200. Therefore, the tip end portion of the welding wire W becomes higher in temperature, making it possible to reduce an amount of energy to be supplied to the welding wire W.
However, the contact 200 provided at its distal end with the insulating guide 218 entails such a drawback that the contact point relative to the wire W becomes unstable. Contrary to an arc welding apparatus where the insulating guide is omitted and the contact point can be kept constant by utilizing the habit of the welding wire being freely bent, the insulating guide 218 provided at the distal end of the contact 200 prevents the welding wire W from being bent just after passing the contact 200, and as a result the contact point between the welding wire W and the contact 200 becomes unstable.
For a satisfactory arc welding, the welding wire portion on the distal end side with respect to the contact point must be heated constantly, with the wire extension length, i.e., the position of the contact point kept constant, however, the welding tip shown in FIG. 1 is insufficient in regard to this point.
In an attempt to stabilize the contact point, the present inventors fabricated a welding tip having a contact 200 whose feeder hole 216 was smaller in diameter (for instance, about 1.23 mm for the wire diameter of 1.20 mm), and carried out arc welding tests thereon, but no satisfactory results could be obtained. It is considered that this is because the feeder hole 216 was worn out early with the passage of the welding wire W therethrough, even though the feeder hole 216 was originally in contact with the welding wire at a constant position.
Thus, the present inventors created an arc welding apparatus in which the feeder hole 216 extended obliquely as shown in FIG. 3. However, such an arrangement cannot sufficiently stabilize the contact point. It is considered that this is because of the following reasons.
To attain a reliable contact between the extreme end of the feeder hole 216 and the welding wire W, the feeder hole 216 must be formed to extend obliquely at a relatively large angle, resulting in a large resistance force at the feeder hole 216 when the welding wire W passes therethrough, thus making it impossible to attain smooth wire supply. Since the welding wire W has a certain rigidity, moreover, the passage of the welding wire causes the extreme end of the feeder hole 216 to be worn out, and hence an urging contact between the extreme end of the feeder hole 216 and the welding wire W is gradually weakened. Furthermore, since the welding wire W is bent steeply at the feeder hole 216, the welding wire portion on the distal end side with respect to the feeder hole 216 gives a significant impact on the insulating guide 218 of ceramic, which impact can be large enough to break the insulating guide. If the oblique angle at which the feeder hole 216 extends is decreased in an attempt to eliminate the aforementioned inconveniences, the contact between the feeder hole 216 and the welding wire W becomes unstable, as in the case of the feeder hole 216 shown in FIG. 1.
As explained in the above, unstable arc is caused by instability in the contact point at which the welding wire is in contact with the feeder. The instability of arc becomes noticeable as the contact being worn out due to friction between the contact and the welding wire.
Another problem is caused by chippings produced while the welding wire passes through the contact. Specifically, such chippings accumulate in the wire insertion hole formed in the welding tip to hinder smooth wire supply.
Moreover, the arc welding apparatus shown in FIGS. 1-3 entails the following problems. Specifically, in an arc welding apparatus having the insulating guide 218 whose distal end is positioned at an axial position short of the distal end of the metal guide 210, spatter S dispersing during the welding adheres to inside the metal guide 210 and further spatter S adheres to the spatter already adhered to the metal guide, as shown in FIG. 4. Thus, the spatter gradually develops and finally narrows or clogs the distal end opening formed in the guide hole 220 of the insulating guide 218, casing a wire supply failure. In addition, gas shielding is deteriorated to permit pits and blow holes to be produced in a weldment.
For the above-mentioned reasons, a welding tip for arc welding is demanded that is capable of achieving stable power supply to a welding wire to produce a stable arc and that is excellent in durability and has a long service life. To achieve smooth wire supply, moreover, accumulation of chippings in a welding tip and adherence of spatter to an insulating guide and a metal guide of a welding tip must be prevented or reduced.